With the continuous decline of available freshwater supplies, the recycling of municipal, industrial, and commercial wastewaters has gained more and more interest in recent years. Accordingly, various membrane-based liquid filtration/separation technologies such as nanofiltration (NF), ultrafiltration (UF), and reverse osmosis (RO) have become ever more important in water treatment applications, such as oil/water emulsion separation and water desalination. Although the commercially available NF, UF, and RO membranes and membrane systems are effective in removing impurities such as small particles, (bio)macromolecules, oily microemulsions, and salts, these membranes often suffer from low flux due to limited permeability. In addition, as the pores of the membranes become clogged (fouled), the flux rate of the membrane decreases during use, making them less and less effective over time. It is therefore desirable to provide higher flux, low fouling membranes for fluid (e.g., water) treatment applications.
Various types of UF, NF, and RO membranes are known, including flat sheet and hollow fiber membranes prepared by phase-inversion or temperature inversion processes using polymeric solutions cast onto porous substrates, and thin film composite membranes prepared by depositing a thin polymer film onto a porous substrate.
In conventional thin film composite membranes, the selective top layer coating has been made primarily by the interfacial polymerization of poly(amino) and poly(acid chloride) monomers onto a porous substrate (e.g., an ultrafiltration or microporous membrane). After Cadotte's pioneering work (e.g., U.S. Pat. No. 4,039,440, herein incorporated by reference in its entirety for all purposes), there have been numerous attempts to improve the performance of UF, NF and RO membranes by interfacial polymerization.
Almost all commercial reverse osmosis (RO) membranes currently used for desalination are composite membranes made by an interfacial polymerization process. Typically, a microporous membrane (e.g., a polysulfone UF membrane) is first soaked in an amine solution. The aromatic amine-wetted UF membrane support is then contacted with one or more crosslinking agents dissolved in an immiscible organic solvent(s) (e.g., trimesoyl chloride in hexane). At the interface of the two immiscible liquids, a dense, crosslinked, and charged polymeric network is formed. Such interfacially polymerized top coating layers typically have a thickness of ˜0.002 to ˜0.3 μm. Current commercial RO membranes have the sodium chloride rejection rate of 99+% and a water flux greater than 35 L/m2 h at a feed pressure of 800 psi.
The majority of commercially available nanofiltration (NF) membranes are also prepared by interfacial polymerization, e.g., comprising a piperazineamide on a microporous substrate. For example, Cadotte et al. (U.S. Pat. No. 4,259,183, herein incorporated by reference in its entirety for all purposes) has successfully demonstrated the fabrication of NF membranes by the interfacial polymerization of piperazine using trimesoyl chloride. These composite nanofiltration membranes exhibited very high MgSO4 rejection rate (99%) but low NaCl retention rate (<60%). Multi-component (piperazine and polyvinyl alcohol, JP 61 93,806; herein incorporated by reference in its entirety for all purposes) and multi-layer coating (sulfonated polysulfone and piperazineamide) composite membranes have also been prepared. For typical nanofiltration membranes, the molecular weight cutoff ranges are from 100 to 5000 Dalton, with a high rejection of divalent ions (>99%) and low rejection of monovalent ions (˜50% or less).
Composite UF membranes have also been prepared by interfacial polymerization. Wrasidlo et al (U.S. Pat. No. 4,902,424, herein incorporated by reference in its entirety for all purposes) prepared composite UF membranes by the interfacial polymerization of a polyethyleneimine-soaked microporous membrane with isophthaloyl chloride and toluene diisocyanate in hexane. The polymerized top coating layer had a thickness ranging from 0.0012 to 0.15 μm, with molecular weight cutoff values ranging from 500 to 1,000,000 Dalton. Stengaard et al (J. Membr. Sci., 53 (1990) 189-202; herein incorporated by reference in its entirety for all purposes) reported reacting an undisclosed aqueous monomer composition with diisocyanates on polyethersulfone UF membranes (MWCO: 20 k˜50 k Dalton). Separation of whey/skimmed milk mixtures were carried out, with a permeate flux ranging from 40˜75 L/m2 h at 30˜60 psi.
Interfacial polymerization methods are suitable for continuous mass production processes because of the rapid reaction kinetics (e.g. poly(hexamethylene sebacamide of high molecular weight has been made in less than 0.02 sec. Condensation Polymers: by Interfacial and Solution Methods, P. W. Morgan, John Wiley & Sons, 1965, and because only few steps are needed to complete the coating process: (i) soaking & squeezing, (ii) applying the crosslinking solutions and (iii) decanting the excess solution and drying. Varieties of reactive monomer selections make it possible to fine-tune the filtration spectrum from reverse osmosis to ultrafiltration range.
However, a major drawback in conventional composite membranes prepared by interfacial polymerization processes is pore blockage in the microporous membrane support when it is soaked in aqueous amine solutions. The blocked pores tend to increase the effective coating thickness of the interfacially polymerized coating layer, and consequently tend to decrease the permeate flux. Also, the chemical nature of polyamide coating (e.g. hydrolyzed acyl halide; carboxylate groups and terminating amine groups), make interfacially polyamide composite membranes more prone to fouling by charged solute species, which also tends to significantly reduce the permeate flux. Typically, before the use of interfacially polymerized polyamide coated membrane in the final step of filtration (NF and RO), the feed solution must be pre-filtered by microfiltration and ultrafiltration in order to keep a stable flux rate without significant fouling.
Nanofibrous supports have been studied for many potential applications, such as biomedical scaffolds and as filters. Because of their small fiber size (diameters of around 100 nm), highly interconnected pore structures, and large void volume (≧60%), nanofibrous sheets can be used as microfiltration membranes (e.g., Gopal et al, J. Membr. Sci. 281 (2006) 581-586; herein incorporated by reference in its entirety for all purposes), but lack the selective coating layer required for UF, NF, or RO applications.
Therefore, there is a need for high-flux UF, NF, and RO membranes having a high permeation rate, high rejection ratio, and a reduced fouling rate compared to filtration systems currently available on the market today. The composite nanofibrous articles of the present invention provide significantly improved performance compared to existing filtration systems, and exhibit high flux rates, excellent permeation rejection ratio and reduced fouling.